There is a scarcity of contiguous amounts of available (unused) spectrum at those radio frequencies which are desirable for wide-coverage terrestrial broadcast communication systems in the high frequency (HF) band, between 3 megahertz (MHz) and 30 MHz, and especially in the low frequency (LF) band, between 300 kilohertz (kHz) and 3,000 kHz. In certain cases, the spectrum of certain conventional information-bearing signals does not entirely occupy a particular frequency allocation, and/or the existing signals may not utilize all of the signaling dimensions, for example, having only amplitude modulation or phase modulation, but not both at once. As a result, it is desirable to generate supplemental signals, especially digitally-encoded signals, in the unoccupied parts of existing bandwidth allocations and/or by making use of underutilized signaling dimensions in order to provide capacity for digital (bit) information in a manner that does not introduce significant amounts of interference to existing signals. The additional information capacity is used for the introduction of new broadcast services, for example, navigation and subscription messaging, and/or the enhancement of existing broadcast services, for example, digital audio broadcasting to provide recovered audio quality comparable to high fidelity tape and compact disc (CD) recordings.
In the United States, the commercial (expanded) AM broadcast band presently occupies a part of the LF band of frequencies between 535 kHz and 1705 kHz, inclusive. The frequency region is subdivided into a plurality of channel allocations with an interchannel frequency separation of 10 kHz (an interchannel frequency separation of 9 kHz allocations has been proposed). Licensed radio stations are subject to restrictions on permissible AM-band signal power, spectrum occupancy (i.e. bandwidth), and geographic location in order to control the expected amounts of interference between stations. Rules and regulations regarding the licensing and operation of AM-band stations are enacted and enforced by the Federal Communications Commission (FCC) in the United States. A restriction for AM-band sound broadcast signals is determined by the FCC "emissions mask". The emissions mask specifies a power spectrum density curve which defines the maximum allowable power of a discrete emission with respect to the licensed power as a function of the frequency offset from the nominal AM-band channel center frequency. Discrete emission compliance at a particular frequency is measured over a 300 Hz integration bandwidth.
Prior art FIG. 1 is a graph which illustrates an emissions mask for AM-band broadcast signals in the United States. The abscissa values are the frequency offset with respect to nominal AM-band channel allocation center frequency 1. The ordinate values are the decibel spectrum magnitudes with respect to the licensed modulated carrier power (dBc). The emissions mask is shown as bold curve 3. Conventional monophonic analog AM signal 7 is the modulated signal representation of the arithmetic sum L+R of the left (L) and right (R) conventional analog audio signals. Conventional monophonic AM signal 7 is substantially confined within, under, and/or beneath emissions mask 3. The center region of mask 3 includes upper inner 5 and lower inner 9 sidebands and occupies a two-sided bandwidth of about 20.4 kHz (i.e. 9.9 kHz+0.3 kHz+0.3 kHz+9.9 kHz) around channel center frequency 1. In the center region, the maximum emission power may be as large as the licensed power; in other words, -0 dBc. Outer sideband regions 13 and 11 are disjoint and immediately adjacent to inner sidebands 5 and 9, respectively, at positive and negative frequency offsets, respectively, with respect to center frequency 1. Upper outer sideband region 13 occupies 9.8 kHz, beginning at a positive 10.2 kHz offset and extending to a 20 kHz offset. Lower outer sideband region 11 also occupies 9.8 kHz, beginning at a negative 10.2 kHz frequency offset with respect to center frequency 1 and extending to a negative offset of 20 kHz. It will be understood by those of skill in the art that the bandwidth occupancies of sidebands 5, 9, 11, and 13 are to change.
In outer sidebands 11 and 13, the maximum power of a discrete emission is attenuated by at least 25 decibels with respect to the licensed power; in other words, -25 dBc. Conventional AM-band broadcasting uses large-carrier amplitude modulation so that a significant fraction of the RF signal power is emitted as the sinusoidal carrier component with center frequency 1. In many circumstances, analog signal 7 may be substantially confined in a spectrum region within inner sidebands 5 and 9, closer to a two-sided bandwidth of 14 kHz (.+-.7 kHz) than 20.4 kHz around center frequency 1, with outer sidebands 11 and 13 substantially unoccupied. There are additional sideband regions defined in emissions mask 3 which are not shown in FIG. 1, beginning at .+-.20 kHz with respect to center frequency 1, which extend significantly beyond outer sidebands 11 and 13.
Since the bandwidth of conventional monophonic AM signal 7 does not occupy all of the spectrum allocated in inner sidebands 5 and 9 and outer sidebands 11 and 13 within emissions mask 3, it is known that one or more supplemental information-bearing signals, preferably representing digital information, may be transmitted together with conventional monophonic analog AM-band signal 7, while still remaining substantially confined within emissions mask 3. When the generation of the supplemental signal is unrelated to conventional analog AM signal 7, the spectrum of the supplemental (i.e. digital) signal is typically confined to outer sidebands 11 and 13 in order to prevent mutual interference between the analog AM signal and the digital signal. In certain circumstances, when the digital signal is related to conventional analog AM signal 7, the spectrum of the digital signal may also include part of or the entirety of inner sidebands 5 and 9. In general, the sideband regions beyond (i.e. at greater frequency offsets than) upper outer 13 and lower outer 11 sidebands are unsuitable for use in broadcasting a supplemental digital signal which provides wide geographic coverage because i) the permissible amount of emitted power in these sidebands is very small, e.g. no more than -35 dBc, and ii) signals at a such a large frequency offset from center frequency 1 may be subject to (or cause) significant amounts of interference from (or to) stations with AM-band transmitters operating on adjacent frequency channel allocations.
In certain applications, it is known that the supplemental digital signal represents a compressed digital audio signal to accomplish the goal of improving the received quality of voice and sound transmissions for broadcasting in the AM-band. U.S. Pat. No. 5,588,022 to Dapper, et. al., describes an In-band On-channel (IBOC) Digital Audio Broadcasting (DAB) communication system for use in the AM-band. In the '022 system, a digital signal, preferably representing a source-compressed high quality audio signal, is combined with a conventional analog AM signal to determine a composite RF signal which is substantially confined within the FCC emissions mask for the AM-band. IBOC DAB makes use of the existing infrastructure of transmitter systems and does not disrupt the economic model for sound broadcasting in the United States. In the '022 system, the composite RF signal is transmitted using a conventional AM transmitter and antenna system so long as the transmission system has sufficiently low distortion and wide bandwidth. A graph representing the spectrum of the signals in the '022 system is shown in prior art FIG. 2, where the composite RF signal spans a bandwidth of 40 kHz and is the additive sum of digital signals generated in upper outer 13 and lower outer 11 sidebands, conventional large carrier analog AM signal 7, and upper inner 5 and lower inner 9 sideband digital signals. Each sideband digital signal is composed of a plurality of narrowband orthogonal sinusoidal subcarriers which are generated and summed together according to the known method of Orthogonal Frequency Division Multiplexing (OFDM), also known variously as Coded Orthogonal Frequency Division Multiplexing (COFDM), Multicarrier Modulation (MCM), and Discrete Multitone Modulation [reference: J. A. C. Bingham, "Multicarrier modulation for data transmission: an idea whose time has come," IEEE Communications Magazine, Vol. 28, No. 5, pp. 5-14, May 1990]. The narrowband subcarriers are each digitally modulated with either binary phase-shift keying (BPSK), quaternary phase-shift keying (QPSK), or 32-ary quadrature amplitude modulation (QAM).
In FIG. 2, groups of OFDM subcarriers are generated to occupy substantially all of upper outer 13 and lower outer 11 sidebands defined under emissions mask 3. These modulated subcarriers have both an in-phase and quadrature signal component with respect to conventional monophonic analog AM signal 7. In the center region of emissions mask 3, the OFDM subcarrier group in lower inner sideband 9 and the OFDM subcarrier group in upper inner sideband 5 are generated as complementary pairs (i.e. each subcarrier in group 5 has a corresponding subcarrier in group 9 with a predetermined phase relationship) so that the resulting composite signal for the inner sidebands has a linear phase-quadrature relationship (i.e. out-of-phase by 90.degree. or .pi./2 radians or positive or negative odd integer multiple thereof) to analog AM signal 7. As a result of the linear phase-quadrature relationship, the spectrum of the composite digital signal in the inner sidebands may substantially overlap in frequency the spectrum of conventional analog AM signal 7. In the '022 patent, conventional analog AM signal 7 is referred to as an I (in-phase) signal, and the composite inner sideband digital signal is referred to as a Q (quadrature) signal. The quadrature composite inner sideband digital signal is also attenuated with respect to the analog AM signal by a significant amount which varies according to each subcarrier in the composite. Since the composite inner sideband digital signal is phase-quadrature to the conventional analog AM-band signal, the two signals may be readily separated from each other in the corresponding receiver system in the absence of distortion using the known method of synchronous linear demodulation. In the '022 system, additional bit information is conveyed by OFDM digital signals generated to occupy upper outer 13 and lower outer 11 sidebands in addition to inner sidebands 5 and 9. However, unlike the composite inner sideband digital signal, the upper and lower outer sideband digital signals have both in-phase and quadrature signal components and are not phase-quadrature to conventional analog AM signal 7. In order to prevent mutual interference between the outer sideband signals and the analog AM signal, the outer sideband signals have to be substantially frequency-orthogonal to conventional analog AM signal 7. The outer sideband signals may be separated from the analog AM signal in the receiver system using bandpass filtering. The correlation processing used in the conventional demodulation of OFDM signals by the Fast Fourier Transform (FFT) mathematical algorithm inherently accomplishes some amount of bandpass filtering.
The premise that a supplemental signal can occupy the same spectrum as conventional analog AM signal 7 by generating the supplemental signal so that it is quadrature to the conventional analog AM signal was known prior to the '022 system, without consideration of whether such signal represents analog or digital information. Various such systems were developed for the application of analog AM-band stereo broadcasting. For example, the analog AM stereo system proposed by Harris [reference: F. G. Stremler. Introduction to Communication Systems. Reading, MA: Addison-Wesley Publishing Company, 2nd edition, appendix H, pp. 679-687] uses linear quadrature amplitude modulation (QAM) to generate two phase-quadrature signals which occupy the inner sidebands in AM-band emissions mask 3; see also U.S. Pat. No. 4,458,361 to Tanabe, et. al., and Y. Sakaie, et. al., "An amplitude-modulated stereo system", IEEE Transactions on Broadcasting, Vol. 26, No. 4, pp. 125-132, Dec. 1980. One of the signals is, or is related to, conventional monophonic L+R analog AM signal 7, and the other signal is, or is related to, an analog representation of the L-R stereo difference signal.
U.S. Pat. No. 4,688,255 to Kahn, describes a system in which a digital signal is generated to be phase-quadrature to the conventional analog AM signal. The digital signal is a narrowband phase-shift keyed (PSK) sinusoidal waveform, whose spectrum includes narrowband emissions at frequency offsets of about .+-.10 kHz from AM-band channel allocation center frequency 1. The spectrum of the digital signal in the '255 system does not significantly overlap the spectrum of the conventional analog AM signal so that compatibility with both monophonic and stereophonic analog AM-band signals is maintained. The amount of power in the transmitted digital signals is varied according to the analog signal power in order to minimize interference in the recovered analog audio signal. A disadvantage of the '255 system is that only a small amount of bandwidth is available for generation of the digital signal. Furthermore, reception of the digital signal is deleteriously affected in the first-adjacent interference circumstance because the upper or lower sideband signals may be substantially occluded, yet both sidebands are always combined in the '255 receiver system.
A known disadvantage of linear phase-quadrature methods for generating a supplemental signal to the conventional analog AM signal, whether representing analog or digital information, is that the existence of the quadrature signal causes distortion in the received and determined analog AM signal estimate in analog AM-band receivers which implement envelope detection. Envelope detection is widely implemented for the demodulation of monophonic analog AM signals because of its simplicity and low cost. Unfortunately, envelope detectors for demodulating monophonic analog AM signals are responsive to both the in-phase signal component, which is desirable when the in-phase signal component is the monophonic analog AM signal, and the quadrature signal component, which is undesirable when the quadrature signal component is the supplemental signal. In order to minimize the amount of distortion introduced by the quadrature signal for envelope detection receivers, the quadrature signal is typically either i) significantly attenuated when compared to the in-phase analog AM signal, as described in the Harris AM-band analog stereo system proposal, or ii) an envelope-correcting factor is introduced which affects to substantially cancel the distortion caused by the quadrature signal in envelope detection receivers, as described in the Motorola AM-band analog stereo system proposal [reference: F. G. Stremler. Introduction to Communication Systems. ibid.]. An advantage of the Motorola AM-band analog stereo system proposal is that the supplemental signal, which is the analog stereo difference signal, may be transmitted at a power level comparable to the monophonic analog AM signal. Comparable power levels are desirable so that the coverage provided by the analog AM stereo signal is similar to that provided by a conventional monophonic analog AM signal. However, the Motorola analog stereo system proposal requires a more complicated receiver system because the effect of the envelope-correction factor has to be removed in order to recover the stereo difference signal.
In the '022 system to Dapper, et. al., the deleterious effect of the quadrature composite inner sideband digital signal on conventional analog AM receivers with envelope detection is mitigated by the significant attenuation of the composite inner sideband digital signal when compared to the analog AM signal. This approach is similar to the Harris analog AM stereo system proposal although the specific method for accomplishing attenuation is different; in the Harris system, a phase offset is introduced between the quadrature signals. Under ideal RF propagation conditions, the low-power digital signal may be separated from the received composite RF signal, which also includes the high-power analog AM signal, with relatively small distortion to the recovered digital signal. However, when the RF propagation conditions are dispersive (e.g. frequency-selective with non-flat group delay) or the transmitter and receiver implementations have appreciable nonlinearities or dispersion, mutual interference between the analog AM signal and the digital signal in the '022 systems may be caused by "cross-talk"; in other words, the loss of the phase-quadrature relationship between the signals, also called the loss of "orthogonality". Since the analog AM signal has substantially larger power than the digital signal, a relatively small amount of crosstalk measured, for example, by equivalent phase error, may result in substantial interference from the analog AM signal to the recovered digital signal.
There are also disadvantages to the '022 system in circumstances where there are other operating AM-band transmitters with the same or similar allocation center frequencies. In order to convey a large amount of user source bit information (e.g. 96 kbit/sec) in a relatively narrow bandwidth (e.g. 40 kHz), the modulation method implemented in the '022 system has to provide for a very high user bit information density, between about 3 bits/Hz/sec and 5 bits/Hz/sec, depending upon the specific bit throughput requirements and the amounts of forward error correction (FEC) redundancy included with the user source bit information. Such high information densities require a relatively large signal-to-noise ratio (SNR) for adequate receiver operation at a sufficiently low bit error rate (BER), typically a BER of less than about 1.times.10.sup.-6 after error correction for digital audio applications. According to published laboratory data for an AM-band IBOC DAB system whose description is similar to that of the '022 system, impairment of the received digital audio signal occurs at a SNR ratio of 19 dB, which is quite large (a large required SNR is undesirable) [reference: "Digital Audio Radio; Laboratory Tests; Transmission Quality Failure Characterization and Analog Compatibility," published by Electronic Industries Association (EIA) Consumer Electronics Group (CEG), Digital Audio Radio (DAR) Subcommittee, Aug. 11, 1995, sections A, AI].
As described previously, the FCC Rules and Regulations act to restrict the power, bandwidth and geographic location of AM-band transmitters with similar allocation frequencies. These rules permit AM-band transmitters with the same channel allocation frequency (e.g. center frequency 1), known as co-channel transmitters, to be situated so that the ratio between desired and undesired signal energies, known as the "D/U ratio", at the edge of coverage for the desired transmitter's signal is only 20:1, which is 26 dB. According to the '022 patent, the power of each OFDM subcarrier in the digital signal is less than -25 dBc (excluding the two innermost subcarriers used for frequency-tracking only). Thus, at the edge of coverage, the interfering analog AM signal power becomes comparable to the digital signal power. Since a relatively large SNR (e.g. 19 dB) is needed for proper operation of the digital signal receiver, having comparable amounts of digital signal power and interfering analog signal power may cause digital receiver failure. As a result, the digital signal may not be reliably received except when the digital signal receiver is in close physical proximity to the desired signal's transmitter; in other words, the coverage of the digital signal is adversely affected when compared to the coverage of the conventional analog AM signal. Signal coverage over a wide area is important to minimize the number of transmitters needed to provide sufficient signal strength over a particular geographic region. A primary goal of IBOC DAB systems, whether operating in the AM-band or the FM-band, is to provide digital signal coverage comparable to the corresponding analog signal coverage.
There are other interference circumstances where the effects of adjacent AM-band transmitters on the operation of the '022 system are even more deleterious than in the case of co-channel interference. Prior art FIG. 3 is a graph which illustrates a circumstance where there is an operating AM-band transmitter under the first-adjacent circumstance. This occurs for (interfering) transmitters with allocation center frequencies at a positive or negative 10 kHz offset from center frequency 1 of the desired transmitter's signal. In FIG. 3, a first adjacent interferer is shown with positive offset center frequency 19, but the choice for illustration purposes is arbitrary. According to FCC Rules and Regulations, for an analog AM receiver located at the edge of coverage for the desired signal, the amount of interfering power in the received signal due to a possible first adjacent AM-band transmitter may be as much as one-fourth (i.e. a D/U ratio of 6 dB) as the desired signal's power. A more optimistic circumstance is shown in FIG. 3, which corresponds to a situation where the receiver is closer to the desired signal's transmitter. The spectrum of interfering analog AM signal 21 significantly occludes both upper inner 5 and upper outer 13 sideband regions in the spectrum of the desired signal, so that reliable determination of unique information in each of these sidebands may not be possible. Lower outer sideband 11 is substantially unaffected. When the interfering transmitter also emits an IBOC DAB signal according to the '022 patent, there may also be interference between lower outer digital sideband 23 of the interfering signal and lower inner digital sideband 9 of the desired signal. FIG. 3 shows only one first-adjacent interferer. In general, there may zero, one, or two or more first-adjacent interferers. However, the probability of two or more first-adjacent interferers causing large amounts of interference at a particular receiver location within the expected coverage of the desired analog AM (and supplemental digital) signal is small. According to the previously referenced EIA CEG DAR laboratory test report, an AM-band IBOC DAB system similar to that described in the '022 patent exhibited impairments in the recovered digital audio signal when the D/U ratio was as high as about 31 dB (.about.35:1) with digital receiver system failure (i.e. muting) at a D/U ratio of about 28 dB. Thus, even a small amount of first-adjacent interference may cause appreciable degradation of the receiver performance in the '022 system.
Prior art FIG. 4 shows an interference circumstance known as second-adjacent interference. Second-adjacent interfering AM-band transmitters are located at frequency offsets of positive 20 kHz and/or negative 20 kHz from desired transmitter's allocation center frequency 1. FIG. 4 shows one interferer at positive 20 kHz offset 27, but in general, there may zero, one, two, or more second-adjacent interferers situated at positive and negative 20 kHz offsets. FCC Rules and Regulations permit the interference power to be about equal to the desired signal power (D/U is 0 dB or 1:1) at the edge of coverage for the desired signal. For the second-adjacent circumstance in FIG. 4, the spectrum of interfering analog AM signal 37 occludes a substantial part of the spectrum of upper outer sideband 13. Correspondingly, the analog AM signal of a second adjacent interferer at a frequency offset of -20 kHz (not shown) would occlude part of lower outer digital sideband 11. If the interfering transmitter also emits an IBOC DAB signal, lower outer digital sideband 38 of the interfering transmitter's signal spectrum substantially occludes upper inner digital sideband 5 of the desired signal's spectrum. According to the EIA CEG DAR laboratory test report, an AM-band IBOC DAB system similar to that described in the '022 patent exhibited impairment of the recovered digital audio signal at a D/U ratio of about 31 dB. In other words, the performance of the digital signal receiver was significantly impaired with an amount of interference that is approximately 35 times weaker than the amount of interference permitted by the FCC Rules and Regulations at the edge of coverage. Thus, the potential coverage of the '022 system is made substantially smaller than the corresponding coverage of the analog AM signal by the presence of adjacent channel AM-band transmitters, which is undesirable. In both the first adjacent and second-adjacent circumstances, the outer digital sidelobes may also cause interference to the recovered conventional adjacent-channel analog AM signal because of the spectrum overlap.
Various methods other than phase-orthogonal linear quadrature amplitude modulation (QAM) for generating a supplemental signal were investigated for the purpose of compatible analog AM-band stereo broadcasting. A proposal by Kahn/Hazeltine described a system in which the left (L) and right (R) conventional analog audio signals are represented by independent upper and lower inner sidebands within the inner sidebands of the emissions mask [reference: F. G. Stremler. Introduction to Communication Systems. ibid.]; see also U.S. Pat. No. 4,589,127 to Loughlin, U.S. Pat. No. 4,124,779 to Berens, et. al., and U.S. Pat. No. 4,569,073 to Kahn. In the Kahn system, the lower inner sideband is the single-sideband modulated representation of the left audio signal and the upper inner sideband represents the single-sideband modulated representation of the right audio signal, or vice versa. Prior art FIG. 5 is a graph which shows the spectrum of such an independent sideband method for analog AM stereo broadcasting. Unlike conventional analog AM signal 7 shown in prior art FIGS. 1-4 in which the analog signal sidebands are even-symmetric around center frequency 1, inner upper 33 (right) and inner lower 31 (left) analog signal sidebands in FIG. 5 are not symmetric, in general. However, it is known that when a conventional analog AM signal receiver is tuned to center frequency 1, then the monophonic (L+R) audio signal recovered in the receiver by envelope detection will be approximately equal to the monophonic audio signal recovered from conventional analog AM signal 7. The implementation of analog AM stereo broadcasting according to the Kahn/Hazeltine system precludes the generation of a supplemental digital signal in the upper inner or lower inner sidebands because all available signaling dimensions are utilized, and there is substantially no unoccupied bandwidth.
Prior art FIG. 6 shows the spectrum of an analog signal according to the Motorola method of compatible quadrature amplitude modulation, which is known by the trademark C-QUAM, for analog audio stereo broadcasting in the AM-band; see also M. Temerinac, et. al., "MF-AM stereo broadcasting: the choice of modulation," IEEE Transactions on Broadcasting, Vol. 25, No. 1, pp. 79-87, March 1989. In a C-QUAM.TM. analog AM-band stereo system transmitter, the analog audio stereo difference L-R signal and analog audio stereo sum L+R signal (i.e. the monophonic audio signal) are first modulated with quadrature amplitude modulation (QAM). The resulting signal is processed by an amplitude-limiter to remove amplitude variations in the resulting signal, and the amplitude-limited signal is subsequently amplitude-modulated by the analog audio stereo sum signal L+R. The presence of the amplitude-limiter results in a nonlinear method of modulation. The resulting signal's bandwidth, as determined by the spectrum occupancy of exemplary C-QUAM.TM. upper sideband 32 and C-QUAM.TM. lower sideband 34 in FIG. 6, is greater than when a substantially linear method of modulation, such as conventional amplitude modulation (AM) or quadrature amplitude modulation (QAM), is implemented. An advantage of the C-QUAM.TM. method for analog audio stereo broadcasting in the AM-band is that the composite RF signal is compatible with envelope detection receivers even when the audio stereo difference and audio stereo sum signals are of comparable magnitude.
The described prior art '022 and '255 systems for generating a supplemental digital signal which is compatible with AM-band analog broadcasting have the disadvantage of a smaller potential coverage when compared to conventional analog AM signals due to external interference caused by co-channel and adjacent channel AM-band transmitters. The prior art '022 system is also susceptible to self-interference due to cross-talk under dispersive RF propagation conditions because of the disparity in analog AM and digital signal power and the substantial overlap in frequency between the analog and digital signals in the inner sidebands. Accordingly, it is apparent from the above that there exists a need in the art of AM-compatible digital broadcasting for: (i) generating a supplemental digital signal which has sufficient bit information throughput for applications such as digital audio broadcasting; (ii) generating a supplemental digital signal whose reception is robust against the effects of frequency-selective distortion and adjacent channel interference; (iii) generating a supplemental digital signal whose reception is robust against the effects of cross-talk under non-ideal RF propagation conditions; and (iv) generating a supplemental digital signal with improved compatibility for analog AM-band receivers with envelope detection.